Who was the Organizer of the family plan? If you were then you simply need to downgrade your plan to individual.
Read this support article about how to downgrade or cancel your iCloud+ storage plan on an Apple mobile device, a Mac, or a PC. --> Downgrade or cancel your iCloud+ plan - Apple Support
- "If you don't have one of these devices, contact Apple Support to change your iCloud storage plan." To contact support, click here --> Choose your country or region - Official Apple Support Select your country, then a product.
If he was the Organizer you can contact Apple about being able to manage the account: For information about how to request access to a deceased family member's Apple account data, and possibly their equipment, read this support article --> How to request access to a deceased family member’s Apple Account - Apple Support
Make sure you take out an individual plan before closing his family plan so you have continuous coverage. If you do not, you may find you lose some content.